Why are TN-C systems not allowed for computer equipment?

[Bavuka]

Hi

Why are TN-C systems not allowed for computer equipment and similar devices?

In addition, the TN-C system:
...
Is forbidden in certain cases:
- Premises with a risk of fire
- For computer equipment (presence of harmonic currents in the neutral)

Source: http://www.electrical-installation.org/enwiki/Earthing_schemes_(_full_page_)#TN_system

After some reading they mention it is because of circulation of harmonic currents in the PEN condcutor, but I am not able to understand what circular path they circulate in?

 

[Windadct]

In computer systems it is expected (required practically) that the neutral will carry significant harmonic current, and should be sized for that purpose. The PE connection - in theory- is reserved for safety purposes only.

 

[CWatters]

I may not explain this very well but...

If all the phases aren't perfectly balanced (or the equipment is single phase) there will be significant currents flowing in the Neutral. TN-C combines the Neutral and Earth onto one wire so any current flowing in the neutral can potentially raise/alter the local Earth voltage (Ground bounce).

The Earth connection on computer equipment isn't only used for safety. Most signals between two computers (or similar equipment, eg printers, plotters, CNC machines) will use differential pair signals to reduce the effect of common mode noise but that might be limited. Some computer equipment may still use single wire connections where local "earth" is used as a voltage reference (bad practice).

It's also common for the wire connecting two "computers" to be screened and for that screen to be connected to the chassis/local Earth at both ends. So any voltage across the red resistor below appears across that screen as well.

Noise on the mains can sometimes include spikes of 1000V or more and separating neutral and Earth functions and avoiding Earth loops can help stop these spikes coupling onto sensitive logic signals (3-5V).

 

[jim hardy]

but I am not able to understand what circular path they circulate in?

3rd harmonic is particularly egregious
if you think about how many milliseconds 120 degrees at line frequency is, 5.555msec at 60 hz or 6.667 at 50 hz,
at thrice line frequency how many degrees do those milliseconds represent ?
at 180 hz, 5.555 msec is a whole cycle .
At 150 hz, 6.667 msec is a whole cycle.

AHA ! Third harmonics in a three phase system are in phase with one another ! Well, you could i guess think of them as 360 degrees out of phase which is indistinguishable from in phase.

So, the sum total of all three individual phase's third harmonic currents returns through the neutral !

That's why you make neutral conductor for computer installations the same size as phase conductors.. Early computers drew power quite rich in third harmonic current. It's because of the huge filter capacitors in their power supplies. Many office buildings had to be rewired with bigger neutrals when every desk got a PC.

There's another reason though why you keep the neutral and grounding conductors separate.
US electrical code stipulates that the grounding conductor must carry ONLY fault current, ie when something is wrong inside an appliance* or a machine. (* An exception exists for electric ranges. )
Normal load current is to return only through the neutral conductor, or of course through another phase conductor as it does in balanced three phase sans harmonics..
The only place you're allowed to join Neutral and Ground is at the service entrance or at the origin of a 'separately derived system' .
Those two remote grounds on the Neutral (that i red X'ed) would be not allowed beause they would let load current caused by unbalance or harmonics get into the green ground wires.

I hope that helps. Search on NEC art 250. "Ground, Neutral, Bond" all have specific meanings in code context.

 

[Bavuka]

Thank you all for helping, I got good answers to my question :)

 

[Averagesupernova]

(* An exception exists for electric ranges. )

Not anymore. For many years the ground and neutral could be shared in clothes dryers and ranges but that has since changed. 4 prong plugs are the norm now.

 

[jim hardy]

Thanks Averagesuper... Good to know that. I'm installing 50 amp receptacle as we speak, so it'll be 4 prongs. .

 

[Averagesupernova]

Thanks Averagesuper... Good to know that. I'm installing 50 amp receptacle as we speak, so it'll be 4 prongs. .

I know you know better but I can't resist telling you to make sure it's also 4 wires. Haha. A good hardware stores electrical section will be able to tell you which 'style' you need.

 

[jim hardy]

I know you know better but I can't resist telling you to make sure it's also 4 wires. Haha. A good hardware stores electrical section will be able to tell you which 'style' you need.

That's fine, and was the plan. I even have forty feet of four conductor #4 to make an extension cord for the welder.

Thanks for being vigilant !

 

[Averagesupernova]

Keep in mind that 240 volt devices that have no neutral current such as a welder do not require a neutral. But ranges and clothes dryers will have neutral current because parts of them run on 120 volts. Don't waste money installing 4 prong receptacles for welder. A 3 is acceptable in that case.